HIV transmission from infected to uninfected cells is known to be more efficient than infection with cell-free virus. In vitro, cell-cell contact stimulates HIV replication 100-10,000 fold. The ability of HIV to utilize and manipulate cell-cell contact for the purpose of efficient transmission critically contributes to the spreading of infection throughout the body and the progression to AIDS. Cell-associated HIV is also thought to play an important role during sexual as well as mother to child transmissions. Despite its importance, very little is known about the molecular mechanisms that govern contact-dependent cell to cell spread and no antiviral therapy has been directed against this step. Here we propose a team science approach to develop antiviral therapies that specifically prevent HIV cell-to-cell transmission. We will take advantage of the first robust cell-to-cell transmission assay allowing the quantitative measurement of virus spreading from cell to cell. We will apply this assay to isolate host factors required for contact-dependent cell-to-cell transmission. Our comprehensive approach will provide the scientific community with novel candidate genes that have the potential to transform research on the prevention of HIV transmission. We will then use the humanized mouse model for HIV infection to directly test if our identified lead candidate genes are required for the establishment of HIV infection in vivo following mucosal delivery. Finally, a parallel and complementary approach we will identify small molecules that function as novel therapeutics specifically interfering with HIV transmission. Our combined genetic and chemical approach will allow the development of novel preventive therapies that reduce the spreading of AIDS. This research proposal will develop antiviral therapies that target the ability of HIV to efficiently spread from cell to cell. It will first focus on basic science by identifying host genes required for HIV transmission, test their physiological relevance in vivo and initiate a translational approach to isolate small molecule inhibitors that specifically interfere with HIV transmission.